Self-Shielded Split Gradient Coil

ABSTRACT

Gradient coil assemblies for horizontal magnetic resonance imaging systems (MRIs) and methods of their manufacture. Some embodiments may be used with open MRIs and can be used with an instrument placed in the gap of the MRI. In general, concentrations of conductors or radially oriented conductors may be moved away from the gap of the MRI so as to reduce eddy currents that may be induced in any instrument placed within the gap. Systems for directly cooling primary gradient and shield coils may be utilized and various coil supporting structures may be used to assist in coil alignment or to facilitate use of an instrument in the MRI gap.

RELATED APPLICATION

This patent application is a continuation of and claims the benefit ofpriority under 35 U.S.C. §120 of U.S. patent application Ser. No.12/951,976, entitled “Self-Shielded Gradient Coil”, filed Nov. 22, 2010,which claims the benefit of priority of U.S. Provisional PatentApplication No. 61/263,280, filed Nov. 20, 2009, entitled “Self-ShieldedSplit Gradient Coil”. The disclosures of each document mentioned in thisparagraph are incorporated herein by reference in their entirety.

BACKGROUND

1. Technical Field

The present application relates to systems and methods for magneticresonance imaging (“MRI”) and particularly to gradient coil assembliesand methods for their manufacture and use with magnetic resonanceimaging systems.

2. Related Art

Magnetic resonance imaging, or nuclear magnetic resonance imaging, isprimarily a medical imaging technique used most commonly in radiology tovisualize the internal structure and function of the body. MRI isdescribed, for example, by E. MARK HAACKE ET AL., MAGNETIC RESONANCEIMAGING: PHYSICAL PRINCIPLES AND SEQUENCE DESIGN (Wiley-Liss 1999),which is hereby incorporated by reference. The present disclosurerelates to the magnetic resonance arts and specifically to horizontal(solenoid magnet) MRIs. It finds particular application in conjunctionwith medical magnetic resonance imaging and will be described withparticular reference thereto. It is to be appreciated, however, that thedisclosure also finds application in conjunction with other types ofmagnetic resonance imaging systems, magnetic resonance spectroscopysystems, and the like.

In magnetic resonance imaging, linear magnetic field gradients are usedfor spatial encoding. Gradient coils are used to produce these linearmagnetic field gradients. Typical horizontal MRIs have horizontalcylindrical gradient coil assemblies that span the length of the device.Horizontal “open” MRIs have been developed for claustrophobic patients.Open MRIs have a split main magnet with a gap in the center andtypically have split gradient coils as well to provide a clear openingfor the patient. More recently, it has become desirable to includevarious treatment and imaging modalities in combination with MRIs andopen MRIs, such as radiation therapy devices, biopsy needles, ablationdevices, surgical devices, ultrasound, PET, SPECT, CT and others. Forexample, it would be desirable to place such instruments in the gapregion of an open MRI. However, problems exist with this placementbecause of interference from various magnetic fields associated with theoperation of the MRI as well as the problem of heating conductors thevicinity of the gradient coil.

Thus, there exists a need for improved MRI gradient coil assemblies andmethods of manufacturing MRI gradient coil assemblies. There furtherexists a need for improved gradient coil assemblies for horizontal openMRIs, and gradient coil assemblies that allow for operation of ahorizontal open MRI in conjunction with additional medical devices.

SUMMARY

Gradient coil assemblies and methods for their manufacture and usewithin horizontal MRIs are disclosed. Some embodiments may be used inconjunction with an open MRI and an instrument placed within its gap.Embodiments of the gradient coil assemblies disclosed herein may includeprimary gradient coils and shielding coils, and may also includeelectrical connections between the primary gradient and shielding coilsthat may be arranged to be a sufficient distance from the open MRI gapso as to reduce eddy currents that may be induced in any instrument thatis placed within the gap.

In some embodiments, the primary gradient coils and shielding coils maybe formed as substantially cylindrical conductors.

In still further embodiments, the conductors in the assembly arearranged so that any concentration of conductors oriented in a radialdirection are located at least 5 centimeters from the gap of the openMRI.

In further embodiments, the electrical connections or conductorsconnecting the primary gradient and shield coils may be located at least5 centimeters from the gap or can be located at the outer ends of theMRI.

In some embodiments, each of the primary gradient coils may be directlycooled and each may also be a hollow conductor.

In some embodiments, the gradient coil assembly may be mounted at boththe outer ends and the gap-facing ends of the MRI.

In further embodiments of the present disclosure, used in conjunctionwith an open MRI, the gradient coil assemblies disclosed herein mayinclude a supporting structure that contains all of the primary gradientcoils and shielding coils and has a portion that traverses the gap ofthe open MRI so that alignment of the coils is facilitated.

In some embodiments, the portion of the supporting structure traversingthe gap may be a portion of an inner former.

In further embodiments, the portion of the supporting structuretraversing the gap may have a radiation attenuation value that isuniform across the structure and may also have a radiation attenuationvalue that is less than five percent for a Cobalt 60 gamma radiationbeam.

In some embodiments of the present disclosure, used in conjunction withan open MRI and a radiation beam aimed through the gap of the open MRI,the gradient coil assemblies disclosed herein may have at least one ofthe primary gradient coils traversing the gap of the open MRI, that atleast one coil having a first radiation attenuation value and theassembly also including a supporting structure that traverses the gapand has a second radiation attenuation value substantially equivalent tothe first radiation attenuation value such that attenuation of theradiation beam will be substantially uniform as it passes through thesupporting structure and at least one gradient coil that traverse thegap.

In some embodiments, the at least one primary gradient coil traversingthe gap may be aluminum. In still further embodiments, the portion ofthe supporting structure that traverses the gap may be impregnated withalumina.

These and other features, aspects, and advantages of the presentdisclosure will become better understood with reference to the followingdescription and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and embodiments of the disclosure are described inconjunction with the attached drawings, in which:

FIG. 1 shows a perspective view of a horizontal open MRI with aninstrument located in its center gap region, as can be used with someembodiments of the present disclosure;

FIG. 2 shows a simplified cross-sectional view of some embodiments ofthe system shown in FIG. 1;

FIG. 3 shows a simplified and expanded cross-sectional view of someembodiments of the gradient coil assembly shown in FIG. 2;

FIG. 4 shows a simplified and expanded cross-sectional view of analternative embodiment of the gradient coil assembly shown in FIG. 2;

FIG. 5A shows a simplified layout of an embodiment of an X gradientprimary coil;

FIG. 5B shows a cross sectional view an embodiment of a conductor usedwith the embodiment of an X gradient primary coil shown in FIG. 5A;

FIG. 5C shows a chart of current paths for an unfolded single quadrantof the embodiment of an X gradient primary coil shown in FIG. 5A;

FIG. 6A shows a simplified layout of an embodiment of an X gradientshield coil;

FIG. 6B shows a cross sectional view an embodiment of a conductor usedwith the embodiment of an X gradient shield coil shown in FIG. 6A;

FIG. 6C shows a chart of current paths for an unfolded single quadrantof the embodiment of an X gradient shield coil shown in FIG. 6A;

FIG. 7A shows a simplified layout of an embodiment of a Y gradientprimary coil;

FIG. 7B shows a cross sectional view an embodiment of a conductor usedwith the embodiment of a Y gradient primary coil shown in FIG. 7A;

FIG. 7C shows a chart of current paths for an unfolded single quadrantof the embodiment of a Y gradient primary coil shown in FIG. 7A;

FIG. 8A shows a simplified layout of an embodiment of a Y gradientshield coil;

FIG. 8B shows a cross sectional view an embodiment of a conductor usedwith the embodiment of a Y gradient shield coil shown in FIG. 8A;

FIG. 8C shows a chart of current paths for an unfolded single quadrantof the embodiment of a Y gradient shield coil shown in FIG. 8A;

FIG. 9A shows a simplified layout of an embodiment of a Z gradientprimary coil;

FIG. 9B shows a cross sectional view an embodiment of a conductor usedwith the embodiment of a Z gradient primary coil shown in FIG. 9A;

FIG. 9C shows a chart of axial position current paths for a half of theembodiment of a Z gradient primary coil shown in FIG. 9A;

FIG. 10A shows a simplified layout of an embodiment of a Z gradientshield coil;

FIG. 10B shows a cross sectional view an embodiment of a conductor usedwith the embodiment of a Z gradient shield coil shown in FIG. 10A;

FIG. 10C shows a chart of axial position current paths for a half of theembodiment of a Z gradient shield coil shown in FIG. 10A.

DETAILED DESCRIPTION

The gradient coil assembly of the present disclosure may be used withany type of horizontal magnetic resonance imaging (MRI) system. It isparticularly well suited for use with a split solenoid or horizontal“open” MRI that includes a gap between two horizontal MRI magnet halves.The gradient coil assemblies disclosed herein are further well suitedfor use with a horizontal open MRI that is used with an additionalmedical instrument being operated within its gap. FIG. 1 depicts such anarrangement with a horizontal open MRI 100 having a gap region 102. Aninstrument 104 is mounted in the gap region 102 on a gantry 110. Alsodepicted are a patient 106 and patient couch 108. In some embodiments,the gantry 110 can be used to reposition the instrument 104 about thepatient 106 (i.e., about the Z-axis shown in FIG. 1).

The embodiment of FIG. 1 can include elements of a system of theassignee of the current application, ViewRay, Inc., described, in part,in U.S. Patent Application Publication 2005/0197564 to Dempsey, titled“System for Delivering Conformal Radiation Therapy while SimultaneouslyImaging Soft Tissue” (hereafter “Dempsey '564”), which is herebyincorporated by reference. For example, the instrument 104 can comprisea radiation therapy device and associated multi-leaf collimator (MLC),which, in combination with a fast-imaging horizontal open MRI, allowsfor improved radiation therapy that accounts for target location duringtreatment, as discussed in Dempsey '564. While only a single assembly isshown as the instrument 104 in FIG. 1, some embodiments can includemultiple assemblies associated with instrument 104. For example, someembodiments may include three radiation head assemblies (not shown inFIG. 1) mounted in gap 102, distributed about the Z-axis, and rotatableabout the Z-axis on the gantry 110. While some aspects of theembodiments disclosed herein are described with respect to the ViewRaysystem disclosed by Dempsey '564, such aspects are not required for usewith the disclosed gradient coil assembly. It is contemplated that thegradient coil assembly disclosed herein may be used in any type of MRI,with or without the use of an associated instrument 104. Furthermore,for systems utilizing an instrument 104, such instruments are notlimited to radiation therapy devices such as radiation sources, or aLINAC, but can include any type of instrument used with an MRI.

FIG. 2 is diagrammatic cross-section of the system shown in FIG. 1. Theembodiment of FIG. 2 depicts a horizontal open MRI 100 including a pairof main magnets 200 a and 200 b, separated by gap 102. The MRI is usedto image a region of interest 202 above patient couch 108. The MRI 100can include additional conventional components not shown, for example,an RF system, including RF coils, and potentially one or more shimcoils. The coordinate system used in the figures and throughout thisdisclosure refers to the longitudinal axis through the MRI bore as theZ-axis. The X-axis extends perpendicular to the Z-axis and from side toside of the MRI 100; the Y-axis extends perpendicular to the Z-axis andfrom the bottom to the top of MRI 100.

An embodiment of the gradient coil assembly 204 disclosed herein isdepicted in FIG. 2 along with its associated coolers 206 a,b andamplifier 208, described in detail below. In this embodiment, thegradient coil assembly 204 is supported by mounts 212 that may belocated both at the outer ends of the main magnets, as well as near theMRI gap. Exemplary mounts are vibration isolating devices such as theIsoDamp® C1002 sold by E-A-R Specialty Composites, a division of AearoTechnologies. Gradient coil assembly 204 may be directly mounted to mainmagnets 200 a,b. However, electrical currents in gradient coil assembly204 in the presence of the main magnetic field (generated by mainmagnets 200 a, b) create torques, forces, and vibrations that can drivevibration and heat into main magnets 200 a,b and increase boil-off.Inclusion of mounts 212 both at the outer and gap ends of main magnets200 a,b can provide for improved vibration isolation and reduce theunsupported span of gradient coil assembly 204.

FIG. 3 shows an expanded, more detailed cross-sectional diagram of anembodiment of gradient coil assembly 204. This embodiment of gradientcoil assembly 204 contains X, Y, and Z shield coils (300, 302, 304respectively) and Z, X and Y primary gradient coils (306, 308, 310respectively). The embodiment also contains connections 312 between eachof the respective coil pairs (connecting X shield coil 300 to X primarycoil 310, Y shield coil 302 to Y primary coil 308, and Z shield coil 304to Z primary coil 306) located at the outer ends of gradient coilassembly 204 (away from the gap 102). The coils 300-310 are disposedwithin a supporting structure 314, which may be made from a materialsuch as an epoxy resin.

As an example, in some implementations of this embodiment, the innerdiameter of the gradient coil assembly 204 can be about 800 mm and theouter diameter can be about 1044 mm, in combination with an MRI 100having a gap 102 that is approximately 200 mm wide and where the lengthbetween the outer ends of the gradient coil assembly 204 is about 2190mm. These dimensions are provided merely as an example and should not beconsidered limiting, as the dimensions may vary.

The disclosed gradient coil assembly 204 may be formed as two separatehalves, so as to leave the gap 102 open for uninhibited physical accessto the patient 106. Such an open configuration allows for use with anadditional instrument 104, for example, a radiation treatment system.However, it is not necessary for the gap 102 to remain completely open,as long as any obstruction does not result in excessive attenuation of,for example, a radiation beam that may be emitted from instrument 104.In the present embodiment, the gradient coil assembly 204 is of singularconstruction, having a gap portion 316 of supporting structure 314. Thegap portion 316 traverses the gap 102 and is a thin, uniform structureconstructed for uniform and minimal radiation attenuation (for example,less than 5% attenuation in the case where a 60Co γ-ray source is used).In the preferred embodiment, gap portion 316 is a portion of acontinuous inner former that can be made from material that is stable inthe radiation environment, for example, an epoxy-fiberglass orepoxy-carbon fiber structure. Its thickness can be, for example,approximately 5 mm, and its density can be, for example, less than orequal to 2 g/cm3. One advantage of this continuous structure is thatopposite sides of gradient coil assembly 204 are naturally aligned,eliminating troublesome two-part gradient alignment issues andassociated asymmetric eddy currents and imaging fields. Anotheradvantage of the instant embodiment is improved mechanical damping andsupport, mechanically balancing and stabilizing the forces and torquesexperienced by the gradient coils in operation. It is contemplated thatgap portion 316 can be provided with access ports cut into it or can beremoved after the installation of gradient coil assembly 204.

FIG. 4 depicts an alternative embodiment for the gap portion ofsupporting structure 400, which contains gradient coils traversing thegap 102. Z gradient coils are naturally gapped, but in this embodiment,primary X and Y coils 308 and 310 can be continuous across the gap 102and can be made, for example, from aluminum (e.g., sheet coils or woundconductors directly cooled). Aluminum is an example of a conductor withlower density than copper and will thus be beneficial in applicationswhere the gradient coil assembly is used with an instrument 104delivering radiation therapy to region 202. An important aspect of thisembodiment is that supporting structure 400 has a radiation attenuationthat matches the attenuation of the included conductors, so thatattenuation will be consistent across the structure. In the case ofaluminum coils, supporting structure 400 could include an epoxy-filamentwound tube with, for example, glass or carbon filaments and appropriatefillers such as alumina between the coils. The uniform attenuation ofthis alternative design will facilitate accurate radiation delivery anddose calculations.

The alternative embodiment of FIG. 4 will have greater beam attenuation(for the case including a radiation therapy device as instrument 104)due to its increased thickness and inclusion of gradient coils. However,this embodiment has the advantage of increasing the usable field of viewof the MRI because it will not experience the loss of linearity ofgradient field strength associated with a split design and will avoidthe problem of radial rollover of the transverse gradient fields.

Turning now to FIGS. 5-10, and with reference to FIG. 3, specificembodiments of the gradient and shield coils will be explained ingreater detail. The present embodiment is described with reference to ahorizontal open MRI having an instrument 104 employed in the gap,although it is understood that many aspects of the disclosure may beapplied to single magnet MRIs used without associated instruments. Theinstrument 104 of the present example can be, for example, the radiationtherapy device previously discussed, although it is understood that manyother treatment or imaging modalities may be used with the presentdisclosure such as biopsy needles, ablation devices, surgical devices,ultrasound, PET, SPECT, CT, LINAC and others.

The radiation device of the present example comprises three equallyspaced Cobalt 60 heads with associated multi-leaf collimators (MLCs),for example, as disclosed in Dempsey '564 and incorporated herein byreference. The MLCs are typically composed of computer-controlledtungsten leaves that shift to form specific patterns, blocking portionsof the radiation beams and shaping them according to a predeterminedtreatment plan. These MLCs are preferably placed close to or within gap102 and are typically made of tungsten with aluminum housings, bothconductive materials. When such materials are placed in the vicinity ofthe time varying currents of gradient coil assembly 204, eddy currentswill be induced in them. Eddy currents induced in instrument 104 willresult in power dissipation in the device and can also interfere withimaging. In the case of MLCs, heat may cause thermal expansion ofindividual leaves and interfere with their operation. Other instrumentsincluding conductive materials that may be used with the disclosedsystem would be faced with similar problems.

The presently disclosed gradient coil assembly 204 reduces these issues,in part, by moving common concentrations of conductors away from the gap102. For example, gradient-to-shield interconnects in horizontal splitgradients are typically located adjacent the radial surfaces of thegradient coil assemblies facing the gap 102. However, the presentlydisclosed gradient coil assembly 204 moves such connectors 312 away fromthe gap 102. For example, the connectors may be preferably spaced atleast 50 mm from gap 102, although the present disclosure contemplatesthat the connectors may be closer than 50 mm from the gap 102. In theembodiments depicted in FIGS. 3 and 4, the connections 312 are locatedat the outer edges of gradient coil assembly 204. Moving concentrationsof current created by radially oriented conductors away from the gap 102will reduce eddy currents induced in any instrument 104. In addition,concentrations of longitudinally oriented conductors or loops areminimized near the gap 102 and the eyes of fingerprint coils arepreferably located at least 50 mm from gap 102. Furthermore, while priorhorizontal split magnet gradient coils have included radially orientedloops along the region of gap 102, the present disclosure preferablymoves any such radially oriented conductors at least 50 mm from gap 102.In the exemplary embodiments disclosed in FIGS. 5-10, the gradient andshield coils are substantially cylindrical in shape. While a cylindricalshape is preferred, the disclosed coils can be other shapes that do notresult in additional conductor and current concentrations near the gap102.

Referring now to the exemplary X gradient primary coil 308 depicted inFIG. 5A, the coil includes 11 forward and 2 reverse turns in each of itsidentical quadrants. The connections between turns (for example,juggles) are not depicted as is common in the art. The centroids of thecurrent paths (all single turns) are shown in FIG. 5C. The X gradientprimary coil 308 includes reverse turns at its ends away from gap 102 tocompensate for thrust and torque forces. Among other things, thiscompensation reduces forces on the thin, continuous gap portion ofsupporting structure 316 (or 400) described as part of this embodiment.The Y gradient coil 310 similarly includes 11 forward and 2 reverseturns as shown in FIGS. 7A and 7C. The shield coils for both the X and Ygradients (300 and 302) of this embodiment include 5 turns in each oftheir identical quadrants as shown in FIGS. 6A, 6C, 8A, and 8C. TheZ-gradient primary coil (306) of this embodiment has 42 total turns asshown in FIGS. 9A and 9C and the Z gradient shield coil has 28 totalturns as shown in FIGS. 10A and C. Additional parameters for someimplementations of the exemplary coils discussed herein are detailed inTable 1 below.

TABLE 1 Example Coil Characteristics Property X Gradient Y Gradient ZGradient Mean radius 414.31 408.21 421.86 (Primary) [mm] Mean radius516.45 510.35 502.80 (Shield) [mm] Conductor 7 mm × 5.1 mm 7 mm × 5.1 mm8 mm × 8 mm Thickness [mm] hollow conductor hollow conductor hollowconductor Number 11 fwd/2 rev 11 fwd/2 rev 28 (total) of Turns (perquadrant) (per quadrant) (Primary) Number 5 5 28 (total) of Turns (perquadrant) (per quadrant) (Secondary) Total Electrical 1533.81 1533.811407.38 Coil Length (Primary) [mm] Total Electrical 1274.74 1264.741533.80 Coil Length Secondary [mm] DC Resistance 68.29 68.28 52.98(Primary) [mΩ] DC Resistance 33.18 33.18 42.09 (Secondary) [mΩ] DCResistance 101.47 101.46 95.07 (Total) [mΩ] Inductance [μH] 227.10224.38 356.325 Non-Linearity +2.81/−3.23/ +2.37/−5.2/ −4.14 [%] over−21.69 −25.46 ±17.5 cm/ ±25 cm/±30 cm Non-Uniformity −33.35 −32.89 −11.0[%] over ±17.5 cm Radial Rollover Z = 0.0 mm 27.7 cm 27.1 cm No rolloverZ = 25 mm 28.0 cm 27.4 cm Z = 50 mm 30.0 cm 29.7 cm Z = 75 mm 34.1 cm33.6 cm Z = 100 mm No rollover No rollover Gradient 16.0 16.0 16.0Strength [mT/m] Current [A] for 528.93 510.40 356.29 G = 16 mT/mSensitivity 30.25 31.348 44.91 [μT/m/A] Slew rate 228.44 237.00 227.76[mT/m/ms] at G = 16 mT/m and V = 1800 V Rise Time [μs] 76.82 73.24 78.71Net Thrust 6.92 4.64 2.34 Force [N] at G = 16 mT/m Net Torque on 79.672.43 0.0 each half [N*m] at G = 16 mT/m Eddy Current <1.0% <1.0% <0.5%Effect (50 cm DSV) Eddy Current 0.42% 0.42% 0.23% Effect Variation (50cm DSV)

To meet performance specifications on the order of those in Table 1,including a 16 mT/m gradient strength and slew rate of approximately 200mT/m/ms, a current driver with high voltage and current capability isdesirable. In the instant embodiment, gradient coil assembly 204 can bedriven with an amplifier 208 such as a Siemens SQ gradient amplifiercapable of delivering 650A maximum current and 2000V maximum voltage,although other amplifiers or multiple amplifiers could be used. The highcurrent and current densities existing in the coils of the presentdisclosure are preferably cooled by direct cooling through the core ofeach coil. The instant embodiment employs separate coolers (206 a and206 b) at each end of MRI 100 and each primary and shield coil is madewith a hollow core as shown in cross section FIGS. 5B, 6B, 7B, 8B, 9Band 10B. The dimensions of the X and Y primary and shield coil crosssections in this embodiment can be the same, for example with a 7 mm by5.1 mm width and height, and a 4.6 mm by 2.6 mm center lumen. Thedimensions of the exemplary Z primary and shield coils can be, forexample, 8 mm by 8 mm with a 6 mm diameter center lumen. While directcooling can be used in the present example, other methods of coolingknown in the art, such as indirect cooling, or a combination of directand indirect cooling, may be used.

While various embodiments in accordance with the disclosed principleshave been described above, it should be understood that they have beenpresented by way of example only, and are not limiting. Thus, thebreadth and scope of the invention(s) should not be limited by any ofthe above-described exemplary embodiments, but should be defined only inaccordance with the claims and their equivalents issuing from thisdisclosure. Furthermore, the above described advantages are not intendedto limit the application of any issued claims to processes andstructures accomplishing any or all of the advantages.

Additionally, section headings shall not limit or characterize theinvention(s) set out in any claims that may issue from this disclosure.Specifically, and by way of example, although the headings refer to a“Technical Field,” such claims should not be limited by the languagechosen under this heading to describe the so-called technical field.Further, the description of a technology in the “Background” is not tobe construed as an admission that technology is prior art to anyinvention(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the invention(s) set forth in issuedclaims. Furthermore, any reference to this disclosure in general or useof the word “invention” in the singular is not intended to imply anylimitation on the scope of the claims set forth below. Multipleinventions may be set forth according to the limitations of the multipleclaims issuing from this disclosure, and such claims accordingly definethe invention(s), and their equivalents, that are protected thereby.

What is claimed is:
 1. A self-shielded gradient coil assembly for ahorizontal open magnetic resonance imaging system (MRI) used inconjunction with an instrument, the open MRI having a longitudinal axisand a first magnet and a second magnet separated by a gap, theinstrument being located within a portion of the gap, the gradient coilassembly comprising: a supporting structure having a first portion and asecond portion, each supporting structure portion mounted inside of arespective first and second MRI magnet; a plurality of shielding coilsdisposed within each supporting structure portion; a plurality ofprimary gradient coils disposed within each supporting structure portionbetween the shielding coils and the longitudinal axis; and electricalconnections between the primary gradient and shielding coils arranged sothat the electrical connections are located a sufficient distance fromthe gap to reduce eddy currents induced in the instrument.
 2. Thegradient coil assembly of claim 1, wherein the electrical connectionsbetween the primary gradient and shielding coils are located at least 5centimeters from the gap.
 3. The gradient coil assembly of claim 1,wherein each of the primary gradient coils are formed as substantiallycylindrical conductors about the longitudinal axis.
 4. The gradient coilassembly of claim 1, wherein each of the primary gradient coils isdirectly cooled.
 5. The gradient coil assembly of claim 4, wherein eachof the primary gradient coils comprises a hollow conductor.
 6. Thegradient coil assembly of claim 5, further comprising two cooling units.7. The gradient coil assembly of claim 1, wherein the MRI has outer endsand the electrical connections between the primary gradient andshielding coils are located substantially at the outer ends of the MRI.8. The gradient coil assembly of claim 1, wherein the MRI has outer endsand gap-facing ends and wherein each supporting structure portion ismounted at both the outer ends and the gap-facing ends of the MRI.
 9. Aself-shielded gradient coil assembly for a horizontal open magneticresonance imaging system (MRI), the open MRI having a longitudinal axisand a first magnet and a second magnet separated by a gap, the gradientcoil assembly comprising: a supporting structure having a first portionand a second portion, each supporting structure portion mounted insideof a respective first and second MRI magnet; a plurality of shieldingcoils disposed within each supporting structure portion, the shieldingcoils formed as substantially cylindrical conductors about thelongitudinal axis; a plurality of primary gradient coils disposed withineach supporting structure portion between the shielding coils and thelongitudinal axis, the primary gradient coils formed as substantiallycylindrical conductors about the longitudinal axis; connectingconductors between the primary gradient and shielding coils; the primarygradient, shielding and connecting conductors arranged so that anyconcentration of conductors oriented in a radial direction with respectto the longitudinal axis is located at least 5 centimeters from the gap.10. The gradient coil assembly of claim 9, wherein each of the primarygradient coils comprises a directly cooled hollow conductor.
 11. Thegradient coil assembly of claim 9, wherein the MRI has outer ends andthe connecting conductors between the primary gradient and shieldingcoils are located substantially at the outer ends of the MRI.
 12. Thegradient coil assembly of claim 9, wherein the MRI has outer ends andgap-facing ends and wherein each supporting structure portion is mountedat both the outer ends and gap-facing ends of the MRI.
 13. Aself-shielded gradient coil assembly for a horizontal open magneticresonance imaging system (MRI), the open MRI having a longitudinal axisand a first magnet and a second magnet separated by a gap, the gradientcoil assembly comprising: a supporting structure having a first portionand a second portion, each supporting structure portion mounted insideof a respective first and second MRI magnet; a plurality of shieldingcoils disposed within each supporting structure portion; a plurality ofprimary gradient coils disposed within each supporting structure portionbetween the shielding coils and the longitudinal axis; and electricalconnections between the primary gradient and shielding coils located atleast 5 centimeters from the gap; wherein each of the primary gradientcoils comprises a directly cooled hollow conductor.
 14. A gradient coilassembly for a horizontal open magnetic resonance imaging system (MRI),the open MRI having a longitudinal axis and a first magnet and a secondmagnet separated by a gap, the gradient coil assembly comprising: afirst plurality of shielding coils located between the first magnet andthe longitudinal axis; a second plurality of shielding coils locatedbetween the second magnet and the longitudinal axis; a first pluralityof primary gradient coils located between the first plurality ofshielding coils and the longitudinal axis; a second plurality of primarygradient coils located between the second plurality of shielding coilsand the longitudinal axis; and a supporting structure containing all ofthe first and second plurality of shield coils and the first and secondplurality of primary gradient coils, wherein the supporting structureincludes a portion traversing the gap for facilitating alignment of thefirst and second pluralities of coils.
 15. The gradient coil assembly ofclaim 14, wherein the portion of the supporting structure traversing thegap comprises an inner former.
 16. The gradient coil assembly of claim14, wherein the portion of the supporting structure traversing the gaphas a radiation attenuation value that is uniform across the structure.17. The gradient coil assembly of claim 16, wherein the radiationattenuation value is less than five percent for a Cobalt 60 gammaradiation beam.
 18. A gradient coil assembly for use with a horizontalopen magnetic resonance imaging system (MRI) and an instrument, whereinthe open MRI has a longitudinal axis and magnets separated by a gap, andthe instrument includes a radiation device producing a radiation beamaimed orthogonally to the longitudinal axis and through the gap, thegradient coil assembly comprising: a plurality of shielding coilslocated at least partially between the magnets and the longitudinalaxis; a plurality of primary gradient coils located at least partiallybetween the shielding coils and the longitudinal axis, wherein at leastone of the primary gradient coils traverses the gap of the open MRI andhas a first radiation attenuation value; and a supporting structurecontaining the shielding coils and primary gradient coils, thesupporting structure having a portion that traverses the gap and havinga second radiation attenuation value substantially equivalent to thefirst radiation attenuation value such that attenuation of the radiationbeam will be substantially uniform as it passes through the supportingstructure portion and the at least one gradient coil that traverse thegap.
 19. The gradient coil assembly of claim 18, wherein the at leastone primary gradient coil traversing the gap is aluminum.
 20. Thegradient coil assembly of claim 19, wherein the portion of thesupporting structure that traverses the gap is impregnated with alumina.